Analog television in North America is broadcast in accordance with the NTSC standard. The NTSC standard calls for about sixty (exactly 59.94) interlaced fields to be presented each second. Each field represents one half the lines or rows of a video frame. Fields are typically referred to as odd and even, with odd field containing odd lines of each frame, and even fields containing even lines of each frame. Thus, in one field half a frame is presented and in the next field the other half of the frame is presented. The human eye perceives the two fields as a single frame.
Most cinema films, on the other hand, are filmed at a rate of twenty-four frames per second. Accordingly, cinema films are converted for viewing on analog NTSC television using a process known as 3:2 pulldown. This conversion is more particularly illustrated in FIG. 1. As illustrated a sequence of film frames ABCDE . . . is divided into even and odd fields, and one field of every second frame is repeated. The resulting field pattern is AOAEAOBEBOCECOCEDODEEOEEEO . . . Thus, every second frame is presented in three fields, and every second other frame is presented for only two fields. Twenty four frames are thus converted into sixty interlaced fields.
In Europe, analog television is transmit in accordance with the PAL or SECAM standards. These standards call for 50 interlaced fields per second. Cinema films are converted for viewing on analog PAL/SECAM television using a process known as 2:2 pulldown. This conversion is also illustrated in FIG. 1. As illustrated a sequence of film frames ABCDE . . . is divided into even and odd fields. Each even and odd field is shown once every second frame. The resulting field pattern is AOAEBOBECOCEDODEEOEE . . . . Twenty four frames are thus converted into about fifty (i.e. forty eight) interlaced fields.
Newer television and computer displays, however no longer display interlaced video. Instead such displays display the video progressively, one line after the next. Accordingly, newer video output devices, such as for example digital versatile disk (DVD) players, computer games and the like, output video progressively, line by line.
Often video to be presented by such progressive scan devices comes from interlaced sources. For example, many DVDs still store video data as fields of MPEG (or MPEG2) data. Moreover, many such sources store the interlaced video in film mode (3:2 pulldown or 2:2 pulldown). Progressive scan devices must be able to accurately assemble progressive scan frames from the interlaced data. They must therefore be able to detect the presence of film mode, to correctly combine frames. To this end, many sources (such as DVDs) are coded with flags that are intended to indicate whether stored video is stored in film mode or not. Unfortunately, video is often poorly edited: video is cut apart and reassembled in a way that destroys the sequence. Similarly, video is often transferred or broadcast without these flags. In short, the flags cannot be relied upon.
As a result, film mode detection circuits are known. One such detection circuit calculates the sum of absolute differences between adjacent fields. If the source video is stored in film mode, the sum will follow a predictable pattern. Such detection, however, cannot distinguish easily between noise, slow movements, content that overlays interlaced and film mode material, high frequencies, and conventional interlaced content.
Other techniques include the analysis of motion vector data from field to field. Such techniques, however, are quite complex.
Accordingly, there remains a need for film mode detection circuit and method that can easily and quickly detect the presence of film mode video.